The following plant-derived plant growth factors are known: barley-derived, fat-soluble fatty acid having a molecular weight of 600 or less (Journal of Plant Physiology, Vol. 121, pp. 181-191, 1985); pine-derived growth factor consisting of oligosaccharides having a molecular weight of 1000 or less (Plant Cell, Tissue and Organ Culture, Vol. 26, pp. 53-59, 1991); carrot-derived, heat-stable growth factor having a molecular weight of about 700 (Plant Science, Vol. 51, pp. 83-91, 1987); and black Mexican maize-derived growth factor which has a molecular weight of 1350 or less, oligosaccharide-like characteristics, and is not adsorbed to either of anion-exchange resin or cation-exchange resin in a buffer at pH of 5 (Journal of Plant Physiology, Vol. 132, pp. 316-321, 1988).
It is difficult to isolate and purify such known plant-derived plant growth factors, and there is no known technique for mass-production of these factors. In order to effectively use a plant growth factor as a plant cell growth promoter, it must be mass-producible.